I. Field of the Invention
The present invention relates to a method of applying a material, preferably a pattern of material on a substrate.
II. Discussion of the Background
During the past decade, soft lithography has developed to a versatile technique for fabricating chemically micro- and nanostructured surfaces [1,2]. Among several techniques known collectively as soft lithography, micro contact printing (μCP) has become the most commonly used method [1]. The technique was initially developed for the transfer of organic molecules [3], but later on also widely used for the transfer of biomolecules such as proteins [4].
Initially polydimethylsiloxane (PDMS) was used as stamp materials in μCP. As a result of the small Young's Modulus conformal contact between the stamp and the target substrate, a prerequisite for the successful pattern transfer, are easily achieved. The drawbacks of soft materials, however, are deformations known as pairing, buckling, sagging or a complete collapse of patterns [5]. PDMS has a Young's modulus of approximately 1 MPa and hence, does not allow to transfer patterns with dimensions below 200 nm. Other PDMS mixtures were investigated to make it harder and thus increase the resolution [6]. The so-called h-PDMS has got a Young's modulus around 8 MPa, enabling the transfer of quadratic patterns with feature sizes down to 80 nm. Line structures with these critical dimensions could not be demonstrated. In addition, even harder materials such as polyolefines, e.g. Affinity [7] or block copolymers SBS (poly(sterene-block-butadiene-block-styrene) or SEBS (poly(styrene-block-ethylene-co-butylene-block-styrene)) [8] were investigated. Their Young's modulus is around 45 MPa and allows the transfer of 100 nm lines. However, the quality of the transfer patterns suffers from sagging effects.
Another field of transfer methods used for the transfer of metals and other solids such as CNTs is nanotransfer printing (nTP) [9,10]. There a thin layer of metal is evaporated onto a patterned elastomeric stamp, which has been fabricated by drop casting of polydimethylsiloxane (PDMS) onto a patterned Si wafer. The evaporated metal layer is brought in conformal contact with an organic layer on a substrate. As a result of the chemical bond formation at the metal-organic interface, the metal-organic adhesion is stronger than the metal-PDMS adhesion and the metal layer is transferred from the PDMS stamp onto the organic layer. The transfer of metal by nanotransfer printing suffers from the same drawbacks of soft stamps as microcontact printing.
The preparation of polymer stamps and subsequent micrcontact printing technique of organic and inorganic structures have been developed. Critical for a good pattern transfer is the conformal contact between the substrate and the stamp. Polymers with different Young's modulus have been used to transfer patterns of organic molecules (proteins and alkanethiols) and inorganic materials with critical dimensions down to ˜75 nm over small areas. For the transfer of patterns with critical dimensions below 100 nm hard substrate and hard stamp materials, at least as far as their contacting surfaces are concerned, are required.
U.S. Pat. No. 6,482,742 [1] and Tan et al. [12], disclose a method of imprint lithography involving the use of fluid pressure to press a mould into a substrate-supported film. According to this method, the mould and/or substrate are sufficiently flexible to provide wide area contact under the fluid pressure. It is emphasized that both substrates and moulds need to have sufficient flexibility and pliability to be used in this method.
However, none of the prior art processes allows and ensures the achievement of a conformal contact between hard surfaces. Accordingly, it was an object of the present invention to provide for a method that can be adapted to the use of hard surfaces within printing processes. Moreover, it was an object of the present invention to provide for a method which allows the usage of hard surfaces so as to enable a transfer of materials, preferably patterned materials with dimensions ≦30 nm. It has also been an object of the present invention to provide for a method that ensures uniform printing over large areas.